The above pages contain all of the rules needed to understand this change.

In 7th edition, there was a rule that characters could never join units while mounted on a flying creature.

There are lots of rules now dictating what units a character can & can't join, but being on a flying monstrous beast is no longer one of them.Can't Join Monsters Can't join Flying UnitsCan't Join Chariot UnitsCan't join SwarmsCan't Join War Machines Can't Join units when riding a monster or if he is a monster

Who benefit from this? Elves that can ride great eagles and Dark Pegasi. Humans can ride pegasi so could also benefit. Lizards that can ride terradons.

I think that covers it. Full Credit where credit is due here - I didn't realize this change until I read a battle report on TheWarhammerForum by Ben Curry from the Bad Dice Podcast where he used this tactic.

He put the unkillable DE Lord on a Dark Peg in a unit of Spears, also with the crown of command. This guy was able to charge out and basically lock down any unit with Stubborn 10 (likely re-rollable) or kill mages / war machines, etc... Lots of utility here. He doesn't get a Look out Sir, but you actually would prefer enemies shoot him instead of the unit (given his un-killablyness).

This is probably the single best of this unit as the magic items available make the elf hard (if not impossible) to kill. But don't let that stop you from trying it!

Yeah but I want to field all fliers! I guess you could put him as the fourth guy in the front rank of Lance. Not sure I'm sold on the Lance formation anymore for anything but Grail Knights (because of their 2 attacks). I.E. in a 5x2 you get 10 Lance hits and 5 horse hits but with a 3x3 Lance formation you get 7 Lance hits and 7 horse hits (assuming no champ). But I digress

Milo - A unit always moves at the same rate as the slowest model, so when the unit moves, charges flees or pursues, always use the lowest movement characteristic and rules of the slowest troop type. pg 99

The flying rules have some interesting language about units that are all flyers can choose to use ground move or fly move to charge.

My guess is ground move when moving with the unit and Fly or Ground when charging out. But that's just a guess.